1. Field of the Invention
The present invention relates generally to memory devices. More particularly, the present invention pertains to analog memory circuits that are capable of long-term memory storage without losing data integrity.
2. Description of Related Art
Sample and hold circuits are a well known and conventional method of storing analog information. FIG. 1 illustrates a circuit diagram of an conventional sample and hold circuit implemented with analog circuitry. One problem with the conventional sample and hold circuit is the leakage current represented by Rleak which causes a non-negligible drift of the output voltage, VOUT. Note that Rleak may also be connected between point H and ground. This voltage drift of a conventional sample and hold circuit may become significant period of a few minutes or even seconds. There have been many attempts in the prior art to overcome this fundamental problem, but, there tends to remain at least some measurable leakage current and usually with significant added cost from additional circuitry.
FIG. 2 is a circuit diagram of an exemplary sample and hold circuit using digital circuit elements according to the prior art. The input voltage, VIN, is input to an analog-to-digital (A/D) converter, and then digitally stored in latch. The digital value can then be reconverted to an analog output voltage, VOUT, by using a digital-to-analog (D/A) converter. It should be apparent that a digital latch circuit as shown in FIG. 2 will hold a number indefinitely, thereby eliminating voltage drift of VOUT. However, a significant amount of circuitry is required to implement the A/D and D/A conversions required, thus, resulting in a large die area and significant cost disadvantages.
FIG. 3 is a circuit diagram of a tunnel injection device in a static analog memory circuit of the prior art, e.g., Diorio et al., “A High-Resolution Nonvolatile Analog Memory Cell”, Proceedings of the 1995 IEEE International Symposium on Circuits and; Systems, Vol. 3, pp. 2233-36, 1995. Although the static analog memory circuit is capable of generating a desired output voltage, VOUT, that will not drift, such a circuit typically must be implemented using non-standard voltages or non-standard integrated circuit processes, making the circuit difficult and expensive to use.
U.S. Pat. No. 5,479,170 to Cauwenberghs et al. discloses a long-term storage dynamic analog memory. The Cauwenberghs et al. device includes a binary quantizer and increment/decrement means for iteratively adjusting the stored analog value to proximity with one of a set of predetermined discrete analog memory levels. However, the Cauwenberghs et al. device appears to require an A/D converter and its associated cost.
Accordingly, there exists a need in the art for a dynamic analog memory circuit that eliminates output voltage, VOUT, drift over a significant period of time, while using standard voltages and integrated circuit processes and minimal circuitry.